Vietnam cities/areas and time to spend

We're planning a trip to Vietnam, via Hong Kong this December. Our
time in Vietnam is not set in stone yet, but we expect to be in the
country between 14-21 days. It seems there are 3 main areas of
Vietnam; northern, central and south My questions are these:

1. How much time should we anticipate in the North around Hanoi and
surrounding areas?

2. Same question for the central area

3. Same question for Saigon and surrounding area.

It seems that we will need to fly from north to south, or visa versa.
Is it cheaper to get the domestic portions of the trip in Vietnam, or
prior to leaving the US?

We are generally city loving people, who also love scuba diving.
Unfortunately, my husband has an ear problem that will keep us above
water this trip, but we are interested in hearing from anyone who has
a "must see" or "must miss" list.

Thanks for your insights.
Ginny